Measuring Velocity

Purpose: measure the speed of the current.

Materials: orange, stop watch, 25' measuring tape (or longer), clip board, paper, pencil

Preparation: measure off 25' (or you can use 50' or 100') along a river bank

Procedure:

• One student throws the orange into the river (near the center where the current is going the fastest), 5' to 10' upstream of the beginning of the 25' section.
• The second student, standing on the beginning of the 25' section and looking straight across the river, yells "go" when the orange crosses his/her "beginning line."
• The third student starts the stop watch at "go" and stands at the bottom end of the 25' section looking straight across the river. When the orange crosses his/her "finish line" the student stops the watch.

Calculation:

• Example: If it took 12 seconds for the orange to float 25', then the velocity is (12feet/25seconds) = 2.08feet/sec

  The velocity at the surface in the center of the river is not the same as the velocity at other points in the river. The velocity is slower as you near the edges of the stream. The velocity is moderate at the surface, faster underneath, then slows almost to nothing as you approach the bottom. By a very, very happy coincidence, in most rivers and streams the velocity in the center at the surface (where you measured it) is just about the same as the average velocity.

 

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